When Jack was one year old his mother bought a packet of $24$ candles for his birthday cake.
That year she put $1$ candle on Jack's cake. When he was two he had $2$ candles and when he was three he had $3$ candles, and so on.
One day Jack's little sister Kate was born. She had $1$ candle on her first birthday cake, $2$ candles on her second birthday cake, and so on.